The present invention relates to a mixture for use in making a sheet of clay which restricts the flow of seawater.
Bentonite is a particularly well-suited clay for use in sheets of the present invention. When wetted, bentonite swells, or hydrates, by absorbing films of water that are thicker than those which form on other clays. The water absorbed by bentonite is retained even when subjected to high pressures. Bentonite is capable of swelling as much as ten to fifteen times its dry volume, and can absorb water to almost five times its own dry weight, while retaining its impermeability.
A very important characteristic of bentonite is that it will swell when uncontained, but will not exert significant pressure when confined against further swelling.
Various references which acknowledge that swellable clay is generally suitable to form water impervious layers in a soil structure. Furthermore, a series of patents issued to Arthur G. Clem, i.e. U.S. Pat. Nos. 4,048,373; 4,103,499; 4,139,588; 4,467,015, suggest that a combination of additives is "absolutely necessary" for the proper performance of a bentonite layer in a seawater environment. The combination which is said to be essential includes a water-soluble dispersant, and a water-soluble polymer. An exemplary dispersant is said to be a salt of phosphoric acid. Alternative dispersants suggested in the Clem patents include salts of alkaline earth metals. An example of the other essential element mentioned in the Clem patents is polyacrylic acid. There is no explanation of the theory behind the selection of these "essential" components of the mixture.
It is believed that the contaminating effects of seawater upon the swellability of a bentonite layer is due to the presence of bivalent ions, such as calcium ions. It is understood that bentonite forms a multi-layer crystalline structure in the presence of water, and in the presence of pure water its ability to swell is at a maximum. However, in the presence of seawater, which may contain various concentrations of contaminants, the ability of the bentonite to form a crystalline structure of maximum volume is inhibited. It is believed that the calcium ions which are contained in seawater have a very significant effect on the ability of bentonite to form a proper crystalline structure It has been found that by significantly increasing the ratio of univalent ions, such as sodium ions, to the number of bivalent ions, such as calcium ions, the ability of bentonite to swell in the presence of seawater can be improved.
Furthermore, contrary to the teachings of Clem, as discussed above, it has been found that the presence of a water-soluble polymer is not essential to the formation of a mixture which will resist the detrimental effects of seawater.
U.S. Pat. No. 4,501,788 (the '788 patent), which is assigned to the assignee of the present invention, shows a particularly useful form in which the mixture of the present invention can be used. The '788 patent describes a flexible sheet comprising a support coated with a swellable bentonite. The support is a porous flexible layer of fabric. Such sheets are used to line ponds and lagoons and landfills, tank farms and hazardous waste sites. By using a sheet of swellable bentonite, an impervious layer is formed to prevent hazardous leachate from entering adjacent groundwater systems.
An important object of this invention is to provide an inexpensive and effective barrier to the flow of seawater through a soil structure or foundation.
Accordingly, a further object is to provide a mixture comprised of the minimum number of components necessary to resist the degradation of the clay layer by seawater.
Still another object of the present invention is to enhance the seawater resistance of sodium bentonite clay with the addition of univalent ions, which will tend to neutralize the contaminative effects of multivalent ions in the seawater.
These and other objects of the present invention will be apparent from the following detailed description of an embodiment of the invention.